FRP, carbon fiber reinforcing plastic (CFRP) in particular, has excellent specific strength and specific modulus, allows manufacture of lightweight structural materials, such as interior material for aircraft, and can therefore greatly reduce fuel cost.
For secondary structural materials of aircraft such as for flaps, fairings and ailerons, and interior materials such as for toilets, ceiling panels, window panels and cabin trunks, in particular, a honeycomb sandwich structure with a honeycomb as the core material and CFRP as the skin material is used in many cases. However, because the carbon fiber is expensive, it can be applied only to a limited extent, even in aircraft where lightweight is most wanted.
These honeycomb sandwich structures are usually manufactured by a honeycomb cocuring method, in which a cloth prepreg is placed on the honeycomb and is heat-pressed to cure the prepreg and bond the CFRP at the same time. However, in the event that a large void exists in the prepreg where carbon fiber does not exist at the place contacting the honeycomb core, such an opening causes a hole in the CFRP skin. To cope with this problem, prepregs comprising conventional 3,000 filament carbon fiber yarn, which is thin and expensive, have been used.
On the other hand, a manufacturing method of a flat yarn woven fabric having uniform fiber distribution and large cover factor, using thick reinforcing fiber thread, has been proposed in JP-A-07-300389. However, this method requires weaving apparatus provided with a weft supply device to prevent twisting and a special apparatus to retain the flatness of the yarn. Furthermore, this woven fabric is unstable because the flatness of yarn is easily lost during the drying step employed when a wet process of manufacturing the prepreg (which is otherwise excellent in resin impregnation to the woven fabric) is employed.
As a structural form stabilizing measure during the prepreg processing, the use of a low melting point polymer has been proposed to maintain the flatness of the constituent yarns. However, the flatness was lost during the wet process for manufacturing a prepreg, resulting in a mesh-like woven fabric with a narrow yarn. Observing how the flatness of yarn is lost during the prepreg process, we found that the flatness of woven yarn can be maintained during impregnation of the resin diluted with a solvent, but a while after the woven fabric enters the drying zone, the flatness gradually begins to be lost.
This invention addresses the abovementioned problems of conventional technology and provides a cloth prepreg of a large cover factor which has uniform fiber distribution, as well as a low cost wet process for manufacturing a cloth prepreg having a large cover factor.